The present invention relates to an electric machine.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Electric machines find application, for example, in the automobile industry and form in particular part of a drive device, e.g. hybrid drive device. The electric machine may operate, for example, as a starter for a drive unit, such as an internal combustion engine, and/or as generator. When a hybrid drive device is involved, the electric machine provides a further drive unit in addition to the afore-mentioned drive unit, with both drive units being operatively connected in such a way as to jointly provide a drive torque, at least temporarily. The drive torque is typically directed to a drive of a motor vehicle. Of course, the electric machine may be used in any suitable way, for example as three-phase machine.
The electric machine includes a driveshaft and a rotor mounted in fixed rotative engagement on the driveshaft. As a result, the rotor is supported by the driveshaft for rotation about a rotation axis, i.e. the rotation axis of the driveshaft. The rotor interacts with a stator which can be disposed in surrounding relationship, at least in some sections, in circumferential direction with respect to the rotation axis. The rotor can have at least one electromagnet and/or at least one permanent magnet. The stator has typically at least one electromagnet. The rotor and thus the driveshaft can be caused to rotate in conventional manner. The driveshaft is typically the output shaft of the electric machine and provides a drive torque of the electric machine. Typically, the driveshaft is supported in a housing of the electric machine via a rolling-contact bearing.
When high rotation speeds of the driveshaft are involved, a lubricating film forms in the rolling-contact bearing, for example between rolling bodies or between rolling bodies and raceways. This lubricating film electrically insulates the rolling-contact bearing so that the driveshaft and thus also the rotor are electrically insulated from the housing. This can cause interference voltage on the driveshaft. This interference voltage, also called shaft voltage, can be in the order of several volts and is the source for bearing currents and EMC incompatibilities of the electric machine or the motor vehicle in which the electric machine is installed.
It would therefore be desirable and advantageous to provide an improved electric machine which addresses these prior art problems and which runs smoothly and with little generation of vibrations while still being reliable in operation over a long service life.